A new ethylene polymerization catalyst system containing a phenoxy-imine ligated V complex and a MgCl2-based compound has been discovered. Unlike ordinary V-based catalysts, the system exhibits higher activities at elevated temperatures, and the activity reached a very high value of 65100kg-PE/mol-cat/h at 75 degreesC, representing the first example of a highly active, thermally robust V-based ethylene polymerization catalyst system.